The recovery of oxygen from air and other oxygen-containing gas mixtures by solid ion-conducting metallic oxide membranes is a rapidly-developing technology with the potential for significant reduction in the cost and energy requirements of oxygen production. Many useful metallic solid oxide materials have been identified in the art which effectively recover oxygen at temperatures in a typical range of 750xc2x0 C. to 950xc2x0 C. Numerous industrial applications which recover oxygen from air are envisioned in which high temperature ion-conducting metallic oxide membrane systems are integrated with gas turbines to enhance the overall energy efficiency of oxygen recovery. In such applications, preheating of the compressed air feed to the membrane system can be accomplished by direct combustion of the compressed air with fuel gas wherein the combustion products pass directly into the membrane modules. Hot, oxygen-depleted, non-permeate gas from the membrane system is expanded through the gas turbine expander to recover useful work.
Solid ion-conducting metallic oxide materials may degrade in the presence of sulfur dioxide at the high operating temperatures required to effect ion conduction, thereby reducing their capacity to conduct or permeate oxygen ions through membranes fabricated from these materials. Because of this problem, the successful operation of ion-conducting metallic oxide membrane systems may require control of sulfur dioxide in the membrane feed gas.
The heat requirements for operating ion-conducting metallic oxide membrane systems typically are provided by the combustion of fuel gas with pressurized ambient air wherein the hot combustion gases provide feed directly to the membrane system. Various types of fuel gases, including natural gas, synthesis gas, and other combustible A gases, contain reduced sulfur compounds such as hydrogen sulfide, carbonyl sulfide, mercaptans, and the like. These sulfur compounds will form sulfur dioxide when the fuel gas is combusted, thereby contributing to the exposure of membrane materials to sulfur dioxide. In addition, ambient air contains sulfur dioxide, and this also will contribute to the sulfur dioxide level to which the membranes are exposed.
The strategy and treatment methods for protecting ion-conducting metallic oxide membrane systems from potentially damaging sulfur dioxide will depend upon several factors, the most important of which are the concentration of reduced sulfur compounds in the fuel gas, the concentration of sulfur dioxide in the ambient air, the operating conditions of the membrane system, and the reactivity of the membrane material with sulfur dioxide. The invention described below and defined by the claims which follow offers a strategy for selecting effective methods to control the sulfur dioxide concentration in the heated feed gas to ion-conducting metallic oxide membrane systems at acceptable levels.
The invention relates to a method for the operation of an ion-conducting membrane system including at least one ion-conducting metallic oxide membrane which divides the system into a feed side and a permeate side, each side having an inlet and an outlet, wherein the method comprises providing a pressurized, heated, oxygen-containing gas mixture which also contains sulfur dioxide, introducing the compressed, heated, oxygen-containing gas mixture into the feed side of the membrane system, transporting oxygen ions through the ion-conducting membrane, withdrawing a hot, oxygen-depleted, non-permeate gas from the outlet of the feed side of the zone, and maintaining the sulfur dioxide partial pressure in the hot, oxygen-depleted, non-permeate gas mixture at the outlet of the feed side at a value below a critical sulfur dioxide partial pressure, pSO2*, which is defined as the sulfur dioxide partial pressure above which sulfur dioxide reacts with the ion-conducting metallic oxide membrane to reduce oxygen flux through the membrane material and below which sulfur dioxide does not react with the ion-conducting metallic oxide membrane to reduce oxygen flux through the membrane material. The value of pSO2* preferably is defined at the temperature of the hot, oxygen-depleted, non-permeate gas at the outlet of the feed side of the zone.
The oxygen-containing gas mixture may be atmospheric air, and the sulfur dioxide partial pressure in the atmospheric air may be defined as the annual maximum, three-hour, time-weighted average sulfur dioxide partial pressure.
The ion-conducting membrane may contain a multicomponent metallic oxide which comprises strontium. The ion-conducting membrane may comprise a multicomponent metal oxide of the general formula (Ln1xe2x88x92xAx)w(B1xe2x88x92yBxe2x80x2y)O3xe2x88x92d, wherein Ln represents one or more elements selected from the group consisting of La, the D block lanthanides of the IUPAC periodic table, and Y; wherein A represents one or more elements selected from the group consisting of Mg, Ca, Sr and Ba; wherein B and Bxe2x80x2 each represent one or more elements selected from the group consisting of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Al, Zr and Ga; wherein 0xe2x89xa6xxe2x89xa61, 0xe2x89xa6y1, and 0.95 less than w less than 1.05; and wherein d is a number that renders the compound charge neutral.
The membrane system may be operated at an average temperature between about 750xc2x0 C. and about 950xc2x0 C. Typically, the value of pSO2* may be in the range of about 10xe2x88x924 to about 10xe2x88x927 atma.
The invention also relates to a method for the recovery of oxygen from an oxygen-containing gas mixture which also contains sulfur dioxide, which method comprises:
(a) compressing the oxygen-containing gas mixture to provide a compressed, oxygen-containing gas;
(b) heating at least a portion of the compressed, oxygen-containing gas to provide a heated and compressed oxygen-containing gas;
(c) introducing the heated and compressed oxygen-containing gas into a membrane separation zone comprising at least one ion-conducting membrane which divides the zone into a feed side and a permeate side, each side having an inlet and an outlet, withdrawing a hot, oxygen-depleted, non-permeate gas from the outlet of the feed side of the zone, and withdrawing an oxygen permeate product from the permeate side of the zone; and
(d) maintaining the sulfur dioxide partial pressure in the hot, oxygen-depleted, non-permeate gas at the outlet of the feed side at a value below a critical sulfur dioxide partial pressure, pSO2*.
The critical sulfur dioxide partial pressure, pSO2*, is defined as the sulfur dioxide partial pressure above which sulfur dioxide reacts with the at least one ion-conducting membrane to reduce oxygen flux through the membrane material and below which sulfur dioxide does not react with the at least one ion-conducting membrane to reduce oxygen flux through the membrane material. The value of pSO2* preferably is defined at the temperature of the hot, oxygen-depleted, non-permeate gas mixture at the outlet of the feed side.
The heating of at least a portion of the compressed, oxygen-containing gas may be effected by combustion of the compressed, oxygen-containing gas with a fuel gas in a direct-fired burner such that the combustion products from the burner provide the heated and compressed oxygen-containing gas, and wherein one or more sulfur-containing compounds are removed from one or more gas streams selected from the group consisting of the oxygen-containing gas mixture, the compressed oxygen-containing gas, the heated and compressed oxygen-containing gas, and the fuel gas.
In this method, the value of pSO2* may be defined by the equation
pSO2* greater than PR[10xe2x88x929YSO2(a)+10xe2x88x926YSO2e(f)/AFR]/[1xe2x88x92FO2]
where PR is the pressure ratio of the feed gas compressor, AFR is the air to fuel ratio in the direct-fired burner, YSO2(a) is the concentration of sulfur dioxide expressed as parts per billion by volume (ppb) in the oxygen-containing gas mixture, YSO2e(f) is the equivalent concentration of sulfur dioxide expressed as parts per million by volume (ppm) in the fuel gas, and FO2 is the fraction of the oxygen-containing gas to the membrane separation zone that is removed as oxygen by permeation through the at least one ion-conducting membrane.
The oxygen-containing gas mixture may be atmospheric air. The ion-conducting membrane may contain a multicomponent metallic oxide which comprises strontium.
At least a portion of the sulfur dioxide may be removed from the oxygen-containing gas mixture, the compressed, oxygen-containing gas, and/or the heated and compressed oxygen-containing gas. The fuel gas may contain one or more sulfur-containing compounds and at least a portion of the sulfur-containing compounds may be removed from the fuel gas prior to the direct-fired burner.
The compressed, oxygen-containing gas may be preheated by indirect heat exchange with the hot, oxygen-depleted, non-permeate gas prior to heating in the direct-fired burner. At least a portion of the sulfur dioxide may be removed from the oxygen-containing gas prior to being preheated by indirect heat exchange with the hot, oxygen-depleted, non-permeate gas.
At least a portion of the sulfur dioxide may be removed from the oxygen-containing gas after being preheated by indirect heat exchange with the hot, oxygen-depleted, non-permeate gas. At least a portion of the sulfur dioxide may be removed from this oxygen-containing gas after being preheated by indirect heat exchange with the hot, oxygen-depleted, non-permeate gas by contacting the oxygen-containing gas with a solid ion-conducting material that has a pSO2* which is less than the pSO2* of the oxygen-selective, ion-conducting membrane of (c).
The oxygen-depleted, non-permeate gas may be heated in a direct-fired combustor to provide a heated, oxygen-depleted, non-permeate gas, wherein the heated, oxygen-depleted, non-permeate gas is expanded in an expansion turbine to generate shaft work. At least a portion of the sulfur dioxide may be removed from the oxygen-containing gas after being preheated by indirect heat exchange with the hot, oxygen-depleted, non-permeate gas by contacting the oxygen-containing gas with a solid ion-conducting material which has a pSO2* which is less than the pSO2* of the oxygen-selective, ion-conducting membrane of (c). At least a portion of the shaft work of the expansion turbine may be utilized to compress the oxygen-containing gas mixture of (a).
If desired, a portion of the compressed, oxygen-containing gas may be withdrawn and combined with the oxygen-depleted, non-permeate gas prior to the direct-fired combustor. A supplemental stream of an oxygen-containing gas mixture that also contains sulfur dioxide may be compressed to yield a supplemental compressed oxygen-containing gas mixture which is added to the compressed oxygen-containing gas after withdrawal of the portion of the compressed, oxygen-containing gas.
At least a portion of the sulfur dioxide may be removed from the supplemental stream of the oxygen-containing gas mixture or the supplemental compressed oxygen-containing gas mixture. The direct-fired combustor may utilize a treated fuel gas obtained by removing one or more sulfur-containing compounds from a raw fuel gas. The fuel gas for the direct-fired burner may be provided by further treating a portion of the treated fuel gas to remove additional sulfur-containing compounds therefrom.